1. Field of the Invention
The present invention relates to semiconductor wafer processing systems and, more particularly, to a gas distribution showerhead for supplying at least two process gases to a reaction chamber of a semiconductor wafer processing system.
2. Description of the Background Art
Semiconductor wafer processing systems generally contain a process chamber having a pedestal for supporting a semiconductor wafer within the chamber proximate a processing region. The chamber forms a vacuum enclosure defining, in part, the process region. A gas distribution assembly or showerhead provides one or more process gases to the process region. The gases are then heated and/or supplied energy to form a plasma which performs certain processes upon the wafer. These processes may include chemical vapor deposition (CVD) to deposit a film upon the wafer or an etch reaction to remove material from the wafer.
In processes that require multiple gases, generally the gases are combined within a mixing chamber that is then coupled to the showerhead via a conduit. For example, in titanium nitride deposition using titanium tetrachloride (TiCl.sub.4) and ammonia (NH.sub.3) as process gases, the two process gases are supplied to a mixing chamber along with respective carrier gases of helium and hydrogen where they are combined to form a gaseous mixture. The gaseous mixture is then coupled through a conduit to a distribution plate, where the plate contains a plurality of holes such that the gaseous mixture is evenly distributed into the process region. As the gaseous mixture enters the process region and is infused with energy, a chemical reaction occurs between the titanium tetrachloride and the ammonia such that the titanium tetrachloride chemically reacts with the ammonia (i.e., the TiCl.sub.4 is reduced by the NH.sub.3) to produce titanium nitride. The titanium nitride is deposited on the wafer in a chemical vapor deposition reaction.
Other two gas chemical vapor deposition reactions include the thermal decomposition of tetradiethylaminotitanium (TDEAT) in combination with ammonia to produce titanium nitride, the thermal decomposition of tetradimethylaminotitanium (TDMAT) in combination with ammonia or a nitrogen-hydrogen mixture to produce titanium nitride, or a reduction of tungsten hexafluoride (WF.sub.6) using hydrogen (H.sub.2) to produce tungsten. In any of these cases and any others that require two or more gases to process a wafer, multiple gases need be uniformly supplied to the process region.
Although it is generally advantageous to mix the gases prior to release into the process region to ensure that the gases are uniformly distributed into the process region, the gases tend to begin reduction, or otherwise react, within the mixing chamber. Consequently, deposition or etching of the mixing chamber, conduits and other chamber components may result prior to the gaseous mixture reaching the process region. Additionally, reaction by products may accumulate in the chamber gas delivery components.
In an effort to maintain the gases in separate passageways until they exit the distribution plate into the process region, U.S. Pat. No. 5,595,606 issued Jan. 21, 1997 (the "'606 patent") discloses a multiple block stack that forms a showerhead that ostensibly maintains two gases in separate passageways until they exit the distribution plate into the process region. As such, the gases do not mix or react with one another until they reach the process region near the wafer.
FIG. 14 depicts a cross sectional view of the prior art showerhead 50 of the '606 patent which includes an upper, a middle and a lower block 58, 60, and 62 having a first set of gas passages 54a, 54b, 54c (collectively, passageway 54) and a second set of gas passages 52a, 52b and 52c (collectively, passageway 52) that branch from the upper block to the lower block in a manner that maintains independence of the passageways. Gas is provided to passageway 52 through port 64 and to passageway 54 through port 72. The passageways 52 and 54 are branched using manifolds 80 and 82 formed in middle block 60. Specifically, passageway 52 is branched through a manifold 80 and passageway 54 is branched through a manifold 82.
A coolant channel 84 is provided in the lower block 62 near the gas outlets 78 for cooling the gas outlets 78 to maintain the showerhead at a temperature below the liquification temperature of a process gas, e.g., below 40 degrees C for TDEAT.
The blocks 58, 60 and 62 are stacked upon one another and O-rings 90 are placed between the blocks 58, 60, and 62 in an attempt to seal the gas within the showerhead. Such O-rings 90 are effective in ensuring that the gas does not leak out of the showerhead but are ineffective in ensuring that the gases do not commingle within the showerhead by leaking between the gas passageways 52 and 54 at the interfaces of the various blocks. Such commingling defeats the purpose of the dual gas passageway assembly, i.e., the gases are not completely separated until they exit the lower block 62 into the process region. Additionally, the existence of O-rings within a process chamber leads to the possibility that the O-ring material will breakdown and contaminate the chamber and/or the wafer surface.
Therefore, there is a need in the art for a showerhead that provides at least two gases to the process region without commingling the gases prior to reaching the process region and without using O-rings to seal the gases into the showerhead.